In the case where it is required to heat and cook food contained within a sealed container made of a plastic or paper material, it is convenient to heat the container with the food contained therein in a wrapped condition. Namely, in general, when the food is heated by means of an electronic oven, particularly a microwave oven, in a wrapped condition, the container is liable to be ruptured by means of the inner pressure which is increased by means of vapor generated during the heating process. In such a case, the food contained within the container is often scattered upon and throughout the interior surfaces of the oven. On the other hand, when the food within the container is heated with the lid of the container removed, the food may be denatured due to the diffusion of the vapors generated during the heating process In order to eliminate this problem, there has previously been provided means for automatically opening the lid of the container when the food sealed within the container is heated by means of the microwave oven. According to the technique disclosed within Japanese Patent Laid-open Publication No. 61-69576, an adhesive layer comprising non-metallic type microwave absorbing particles is applied to the lid of the container. In addition, according to the technique disclosed within U.S. Pat. No. 4,210,674, an electrically conductive material is applied to the lid of the container and the composition of the conductive material is designed so as to absorb the energy. According to these methods, the lid is heated and partially melted so as to be opened as a result of the provision of the microwave absorbing material.
In connection with the use of electronic or microwave ovens, a standing wave is generated within the oven casing and, hence, the strength of the microwaves is not uniformly distributed. With the methods of the conventional techniques described above in which the microwaves are absorbed and the lid is opened as a result of the heat generated by means of the heat generating material or element, in a case where the heat generating material is applied upon a portion of the bid at which the strength of the microwave is weak, there may result a case wherein the lid is not opened due to inadequate heating thereof. In the case where the heat generating material having a large size is utilized so as to open the container lid within the low strength region of the microwave, and if the heat generating material is positioned within the high strength region of the microwave, a widened opening may be formed within the lid of the container, through which a large amount of the vapor will be dispersed whereby the food will be denatured.